This invention relates to systems and methods for controlling and analyzing data storage systems, and more particularly, to systems and methods for controlling and analyzing performance and physical attributes of sealed disk drive units using an externally accessible but internally integral positioning system.
In the present state of the art pertaining to digital storage on movable media, extremely high track densities (of the order of 1000-2000 tracks per inch) have become feasible. Workers in the art have constantly sought to increase the number of usable tracks per disk, i.e. the track density, since it is intuitively recognizable that doubling the number of tracks per inch effectively doubles the amount of stored data. As track densities were increased, it was soon found that open loop systems, such as incremental positioning systems without actual head position feedback, could not meet the accuracy requirements. Such previously minor factors as thermal errors and manufacturing tolerances could not be accepted because of the very small track spacings. Consequently, developers went to embedded servo track systems, and sophisticated dedicated servo surface systems which may also be combined with some embedded servo track information. Embedded servo track systems have prewritten reference patterns within the data tracks which enable precise location of magnetic heads for writing and reading during operation. Dedicated servo surface systems have a single disk surface on which there is only recorded head position information. This information is continuously read in order to position the remaining read/write heads mounted on the common actuator. The constant quest for ever higher track densities has led to modern systems which have 2,000 to 4,000 or more tracks per inch, for magnetic media.
Some of the techniques heretofore used have written the required reference patterns by accurately positioning the head assembly by a separate apparatus which is typically not part of the final drive assembly. For example, an external rotary or linear actuator may be attached to the head assembly drive actuator. With the position of the external actuator being accurately controlled, the head assembly actuator correspondingly follows. This, however, can result in positioning inaccuracies due to various factors such as the friction in the coupling between the two actuators, the relative position of the external actuator and the servo head, and errors between the external position and actual internal head position.
A substantially more costly technique, but one which may be adequately precise, mounts a retroreflector assembly on the head assembly actuator, and directs reflected light to a fixed laser interferometer, which can give an extremely precise position signal for the head actuator in the form of a distance measurement to the retroreflector. Then, an external actuator can be used to position the head drive, or the drive's head actuator can be placed in a servo loop which uses the interferometer for position feedback. This expedient is, however, expensive and as usually practiced, involves an incompletely assembled drive, and clean room operation.
A serious disadvantage is the fact that the disk drive unit must be in an incomplete stage of assembly in order to obtain access to the head assembly actuator, and thereafter maintained in a clean room for the servo track writing operation. A clean room is required to provide the equivalent environment of a clean sealed disk drive assembly. A purified atmosphere without particulate contaminants is needed to "fly" the magnetic head assembly at the necessary close spacing (of the order of a few microinches) to the surface of the disk. Otherwise, particulates, such as ordinary dust, appear as relatively massive objects that affect both the head dynamics and recording.
It is evident, therefore, that none of the available expedients provides a suitably low cost, high precision, method for positioning and writing position reference information on a disk storage medium, or particularly for doing so with a sealed type of drive and under conditions and with procedures suitable for mass production operations.
There are circumstances in which it is desired to sense or control a part of the dynamic interior mechanisms of an open or a sealed disk drive, in terms of determining where the head mechanism is, or the angular position of the disk itself, or both. In preparatory testing of a newly assembled disk drive unit, it may be desirable to analyze the quality of the write/read circuitry and the media consistency without the need or use of servo tracks. For example, testing data amplitude, modulation or pulse width at a varying radii along the rotatable medium can identify defects in the recording medium and or a malfunctioning write/read head. Without a full operating system and servo information, this cannot redily be readily accomplished, if at all, by present techniques.
As a different example, it may be desirable to establish off-track or cross-track performance. To accomplish this, a track of data can be written and subsequently read from positions which are at small increments located increasing further away from the track location, wherein the increments are much smaller than the standard prescribed distance separating two concentric tracks of data.
Likewise, the head assembly can be positioned so as to write first and second tracks of data separated by a predetermined distance. The first or second track of data can then be read at small increments in the gap therebetween, moving from one track towards the other, so as to ascertain the cross-track interference between the first and second tracks. With present techniques utilizing servo information, this cannot readily be accomplished without a fully functioning system.
Yet still another advantage is the ability to determine the magnitude of any unwanted bias torques acting upon the rotary actuator as well as the magnitude of system friction without having a fully operating system. Again, present positioning techniques do not lend themselves to such measurements.